Method of preparing paper pulp



July 12, 1960 A C- KIBRICK ET^L 2,944,928

METHOD OF PREPARING PAPER PULP Filed Sept. 20, 1957 H ...mIlmm Mnrnon orranrannso PAPER PULP rated sept. zu, 1957, ser. No. esausti 6 claims.(ci. isz-az) This invention relates to the manufacture of wood pulp suchas that used in paper manufacturing purposes, and

in particular is concerned with improvements in the kraft `or alkalinesulfate paper process.

Hitherto, in the preparation of woodV pulp for paper production, wood,in the form of finely divided pieces, is subjected to a cooking processin the presence of sodium sulides and minor amounts of sodium sulfates.It is, for example, customary to treat raw wood chips, such as may beobtained from southern yellow pine, notably mixtures of loblolly,longleaf and shortleaf pine, and also from jackpine and from variousspecies of spruce, iir, hemlock, larch and aspen, with a liquor preparedfromV sodium hydroxide and sodium sulfide. These agents are combined insuch amounts as to provide a solution having a predetermined alkalinityand suliidity, and are charged into a digester together with anappropriate quantity of raw wood chips. The cooking or digesting step isaccomplished at moderately elevated temperature and pressure. At the endof this stepthe cooked wood pulp` together with the accompanyingresidual liquors is introduced into a separator where by means voffiltering and other` known means the digested vwood pulp, comprisingchiel-ly cellulose, is isolated. The liquors are recycled backto thedigestion stage after various treatments involving elimination of thelignin, and adjustment of the alkali and sullidity factors. Theseparated Wood pulp may be washed, screened, bleached, dried andultimately employed in ,the fabrication of paper products.

The wood used in the process is usually first debarked, and cut andchipped to appropriate size. The types used vary considerably butgenerally show properties falling within the ranges shown in thefollowing table:

TABLE I minimum maximum Y Specific gravity L 0.31 0.56 hrinkage 7. 914.1 Bark VV7. 18. 9 Cellulose 54. 4 63. 0 Lgillll 25. 9 34. l.

2,944,928 Patented July 12, 1960 'TABLE II KMnO4 Approx- Y Type Number1n e End Use Yield lim- 18 46 Bleached bags and writing, wrapping andprinting papers.

B 24 48 Unbleached heavy papers for bags,

wrappngs, etc.

C 34+ 50+ Uibleaiched corrugated and solid-liber oar In the digestion ofthe prepared Wood the factors entering into consideration are thespecies and quality of the wood and the characteristics desired in thelinal product. Time and temperature ofthe digestion step, suliidity ofthe cooking liquor, the ratio of the cooking chemical to the wood used(on a moisture free basis), and the concentration of the cookingchemical in the liquor may vary widely. It is known that the process maybe carriedY out either in batch or in continuous digestion systems. Itis further known that the black liquor may be discarded or recycled backinto the process. VThese andf other factors understood by those skilledin the art are` related to the ultimate characteristics of the pulp.

Upon completion of the digestion step the liquors and solids arerecovered and the solids are leached with water. The pulp is separatedand subjected to further processing. The weak black liquor may beconcentrated and/or recycled back to the digestion process where it maybe reinforced with make-up chemicals. Hydrogen sulfide is partiallyremoved in the evaporators as a result of the following reaction:

-I- HBS In the direct heat evaporators hydrogen sulfide is readilyevolved, but not completely in most operations, as a result of thecarbonation reaction which occurs as a result of the direct contact withthe flue gases. 4The resulting strong black liquors are-decomposed inthe recovery furnace, where make-up salt cake also may be added, and theresulting `smelt dissolves to produce the so-called green liquors. Uponcausticizing and clarifying this liquor a so-called white liquor isrecovered which may be used to digest incoming wood chips, thus com-Nazsa-HzoNaHsLj-Naon Plants operating on the sulfate process normallyfind that a relatively well-defined equilibrium of composition andsullidity can be established. The source of sulfur make-up is throughthe use of salt-cake. While from one pulp mill to another Widevariations may exist-it may be stated generally that sodium monosulfideand sodium hydrogen Vsulfide are the agents predominantly responsible,

for thesuccess of the process. 4

Figures 1 and 2 demonstratethe advantages of the polysulfide pulpingprocess.

Hitherto the yield of wood pulp obtained in the sulfate process has beenwithin the range of about 46% to 50% when producing pulps havingpermanganate values between about 18 and 34 respectively. We now havefound that the yield of wood pulp may be significantly increased ashereinafter described:

As originally conceived our studies were directed to establishing theeffect of polysulfdes with reference to the cellulose and lignins inwood. In this view it was found that difiiculties were encountered inobtaining sodium polys'ulfides,'and therefore sulfurated potash wassubstituted on the theory that the potassium ion would in generalconstitute the equivalent of sodium. We found however, to ourconsiderable surprise, that the substitution gave an unexpected increasein the pulp yield ranging from 1 to l15% over and above the amountsecured in control runs employing conventional sodium monosulfide orsulfate systems. This unusual result was first ascribed to the presenceof the potassium ion. However, further investigation indicated that theyield increase was toV a considerable extentattributable to the presenceof alkali polysulfides. However, we do not wish to be bound by anytheoretical explanation.

Our process vcontemplates they introduction of chemicals to the body'ofraw wood chips in ratio of about 4 to 1, based on the amount of cookingliquor and dry basis .wood chips, and carrying out the digestion of thewood under elevated temperature yand pressure. This digestion iscarried. out in a pressure vessel or autoclave preferably, with.circulation of the cooking liquors, at a temperature ranging from about160 C. to 180 C. under autogenous pressure which may normally range fromabout 100 to V140 p.s.i. The chemicals and the wood chips being admixedthe vessel after suitable venting may be closed and the temperatureraised to about 170 C., or normal operating temperature, the period oftime for such increase in temperature consuming about 0.5 hours to 5hours, about 1.75 hours being considered normal. After the charge hasattained the desired temperature, it may be held at about thattemperature for 0.5 to 3.0 hours, preferably about 1.0 to 1.5 hours. Asis understood by those skilled in the art, an increase in temperaturenormally shortens the period of time required to produce a pulp of givencharacteristics.

We have found that by carrying out the digestion process underconditions herein disclosed the wood pulp product of given permanganatenumber may be obtained in higher yield. We believe that the cause ofthis higher yield can be ascribed to the action of the chemicalsinvolved in the digestion process, specifically, in. that the attackthereof on the wood material, especially the cellulose component, isless severe. It therefore becomes possible pursuant to our invention tomodify the concepts of optimum operatingconditions that have hithertoprevailed. It may, for example, be desired to obtain a given yield inshorter time by operating at higher temperatures, or the process may beoperated commercially at lower temperatures and shorter times -to give ayield equal to that now produced in conventional sulfate operations.Depending upon the amount of delignification obtained, our improvedprocess makes possible yield increases ranging from about 1 to 15%,normally between about 4% and about 10% in comparison with conventionalsystems, these yield increases being equated to a base of 100%. Ingeneral we find that the yield increase in the case of severe cooking,such as may be used to produce bleachable grades, is notably higher thanhitherto, and that in methods usingrless severe cooking, the observedimprovement in yield generally becomes even more pronounced.

In carrying out the cooking operation the amount of active alkaliexpressed as NazO may vary between about to' about 35 percent based onthe charge of Wood used. Good results may be obtained using about 20 to25 percent. Sulfidity which is normally expressed as a percentage of theactive alkali used, may be established by introducing into the so-calledwhite liquors, containing NazS, alkali polysuldes such as NazSz, Na2S3,Na2S4, Na2S5, or possibly higher polymers, and mixtures thereof, theseagents being generally referred to as Na2S.l The employment of thesodium system has some advantage from the standpoint of cost. However,assuming equal cost, operations employing sulfurated potash or sulfidepolymers of potassium are preferred.

Since the atomic weight of potassium exceeds that of sodium it would beexpected that the effective amount of active alkali as K2O would beproportionately higher than Na2 O. It was found, however, that whenequal weights are compared, for example, using an active alkali weightpercent of about 25 percent, of KOH, and when using sulfurated potash,as against NaOH, the potassium system gave the higher yield of pulp. l

In preparing the polysulfides, elemental sulfur as in the form offlowers of sulfur may be reacted with the alkali in the form of thecarbonate, hydroxide or monosulfide. Commercially available sulfuratedpotash is produced by reaction of K2CO3 with elemental sulfur at atemperature of about 350 C. Although some oxygensulfur intermediatecompounds exist in the commercial material it is undoubtedly thepolysulfides or sulfide polymers that constitute the active agents inthe pulp yield increase. Various methods of preparing sulfide polymersof the type suitable in our process are known to the art. The presenceof the polymers may be visually observed by noting a darker color of thesolution, approaching red-brown. In the practice of the process theprepared liquorsfor digestion of the wood normally contain in additionto the sulfide polymers, intermediate compounds having an oxygencontent, such as the thiosulfates. Polysulfide solutionsin the absenceof air are relatively stable at room temperatures, and at highertemperatures, especially abovev C., undergo a degree of decomposition.Thev sulfidity of the treating liquor-may be yregulated within the rangeof l0 to 30, 19 percent being considered preferable using sulfuratedpotash, In this respect it should be noted the sultidity figures arebased on the conventional iodine-thiosulfate titration, and that otheranalytical methods may give-values somewhat at variance, as will beAunderstood by those skilled in the art.

Upon completion of the digestion in the manner set forth the digestionmass is discharged into a blow tank where disintegration of the cookedwood chips and the solids recovered therefrom .takes place. These solidsupon leaching and screening are refined and then processed into paperproducts, following a bleaching operation when bleached pulps aredesirable. Pulps prepared according to our preferred process makepossible the luse of a less severe bleach and the use of less severebleaching chemical, such as calcium hypochlorite.

The liquor separated from the wood pulp, known as Weak black liquor,is'subjected to evaporation whereby there are removed water andsulfides. Hydrogen sulfide evolved from these and thek blowdownoperations may either be discarded or converted to sulfur available forrecycling to prepare additional treating liquor containing polysulfidesor sulfide polymers. The concentrated black liquor, which may contain 50to 70 percent solids may be admixed with additions of` alkali sulfates,such as Na2SO4 and/or K'2SO4 to make up lost alkali,Y and decomposed ina furnace at a temperature of aboutr1000 C. or higher. Carbonaceousmaterial resulting from this treatment may be recovered and used as afuel or as a reducing agent for the alkali sulfate. While KZSO., has ahigher decomposition temperature than Na2SO4 it is found that underreducing conditions a temperature of about 1000 C. is suitable in mostcases. Further, since KZSO.; has a higher decomposition temperature thanNa2SO4, less KZO loss may occur in the smelting operations than in thecase'of the sodium compounds.

. as in the foregoing examples.

. percent.

The smelt from the furnace' containing predominantly alkali sulfide,carbonate and unreduced sulfur compounds, is cooled with water anddissolved. Preferably a portion or all of the cooled smelt may betreated with sulfur for formation of alkali polysuliides. Part or allofthe polysuldes prepared may be dissolved and clarified prior to usefor digestion with fresh Wood. The polysulfides may be combined in anydesired proportion with the treating liquor or `so-called white liquorcontaining monosuliides. As above noted, sulfide polymers also may beprepared by treating a portion or all of the white or green liquors withelemental sulfur, and if desired the polysulde content may be furtherraised by additional sulfur. Sulfur also may be mixed or slurried withthe white liquors prior to or during the reaction' with the wood chips.

In further illustration of the invention the following specic examplesare given:

Example I To 100 pounds (moisture free basis) of Wood chips, composed ofapproximately equal amounts by Weight of loblolly, shortleaf andlongleaf southern pine, there was added sucent alkali, alkalipolysuliides and water to provide a ratio of water to wood (moisturefree basis) of 4, an active alkali content of 28.0 percent, expressed asthe hydroxide, and a suldity of 20.5 as determined using theiodine-thiosulfate titration method. All of the sulfur was added in theform of a solution prepared by dissolving the previously prepared alkalisulfide polymers. The materials, having been introduced into a pressurevessel, the later was closed, and with intimate mixing of the solid andliquid phases, the mixture was gradually heated over a period of 100minutes to 172 C. This temperature was maintained at autogenous pressurefor an additional period of 100 minutes, at the end of which time thecontents of the pressure vessel were discharged to a collection deviceat atmospheric pressure. The solids were removed and recovered byfiltration and then washed by repulping and mixing with three 300 gallonportions of water at a temperature of about 35 C. Solids from the lastleaching were recovered and weighed with a product weight of 52.0 pounds(moisture free basis) unscreened yield. Chemical analysis of the productrevealed the KMnO4 number to have a value of 24.6. The product wassubjected to conventional refining treatments and provided a papermaterial having physical properties comparable to those'of conventionalsulfate pulp.

Example II Following the general details of procedure of Example I,sulfurated potash was used. The active -alkali content was controlled to28 percent, the sulfidity to 20.5 percent, and the temperature ofdigestion to 178 C. The product Weight was found to be 49.1 percent(moisture free basis) unscreened yield, and 48.7 percent screened yield.The KMnO4 number was 17.95.

Example III The same general details of procedure were followed Theactive alkali value was controlled to 28 percent KOH, the suliidity to20.5 The temperature was raised to a maximum of 168 C. during a periodof 70 minutes and thereafter maintained at that point for an additional70 minutes'. There resulted an unscreened yield of 58.6 percent of aproduct having a KMnO4 number of 88.6.

Example IV The procedural details hitherto set forth were carried out,except that a conventional sulfate system with 25 percent NaOH activealkali was used. Suldity was controlled to 20 percent. The temperatureof the reaction mass was brought overa period of minutes to atemperature of 172 C., and so maintained for an additional 100 minutes.The unscreened yield was 46.9 percent, the screened yield, 46.2 percent.The KMnO4 number of the product was 18.2.

Example V In this case the process described in the above examples wasmodified in that a 34 percent KOH active alkali system was established,at 20 percent sulldity. The digestion was elfected during 100 minutes to172 C. and a further 100` minutes, temperature held substantiallyconstant at 172 C. The unscreened yield of product was 47.0 percent; thescreened yield 46.6 percent. The KMnO4 number was 18.4.

From the foregoing description and illustrations further details withinthe scope of the present invention will be apparent to those skilled inthe art. We claim and desire to protect by Letters Patent:

1. In the method of producing wood pulp according to a pulping process,the improvement comprising cooking the raw wood in solutions of alkalipolysuldes with a sulidity of 10% to 30% maintained by alkalipolysuldes.

2. In the method of producing wood pulp according to a pulping process,the improvement comprising cooking the raw Wood in solutionspredominately of alkali polysuldes.

3. In the method of producing wood pulp according to a pulping process,the improvement comprising cooking the raw wood in solutionspredominately of alkali polysuldes, with the suldity of 10% to 30%maintained in whole or in part With alkali polysuldes.

4. ln the method of producing wood pulp according to a pulping process,the improvement comprising cooking the raw wood in solutions ofpredominately alkali polysuldes with the sulidity of 10% to 30%maintained with a combination of alkali polysuldes and the conventionalsodium monosulde.

5. In the method of producing wood pulp according to a pulping process,the improvement comprising the introduction into a spent cooking liquorcontaining NazS, alkali polysuldes including Na2S2, Na2S3, Na2S4, NaZSS,and higher polysuldes and mixtures thereof, these agents generallyreferred to as NazSx, to maintain the suldity of 10% to 30% with thesepolysuliides, and cooking the pulps with the liquor high in alkalipolysuldes content.

6. In the method of producing wood pulp in the pulping process, theimprovement comprising the adding of concentrated alkali polysulides toa spent liquor to produce a suldity of 10% to 30% and pulping with themixture.

References Cited in the le of this patent UNITED STATES PATENTS1,689,534 Richter Oct. 30, 1928 2,007,024 Richter July 2, 1935 2,747,995Hooper et al May 29, 1956 OTHER REFERENCES Pulp and Paper Manufacture,vol. I, Preparation and Treatment of Wood Pulp, 1950. page 374.

1. IN THE METHOD OF PRODUCING WOOD PULP ACCORDING TO A PULPING PROCESS,THE IMPROVEMENT COMPRISING COOKING THE RAW WOOD IN SOLUTIONS OF ALKALIPOLYSULFIDES WITH A SULFIDITY OF 10% TO 30% MAINTAINED BY ALKALIPOLYSULFIDES.